Xylose Lysine Deoxycholate (XLD) Agar

🧫 Xylose Lysine Deoxycholate (XLD) Agar Interactive Learning System

Intended Use

XLD agar is a selective and differential medium for isolation of Salmonella and Shigella from clinical and food samples.

Principle

• Deoxycholate inhibits Gram-positive bacteria.
• Xylose fermentation differentiates enterics.
• Lysine decarboxylation restores alkaline pH.
• H₂S production forms black centers.

Composition

Component	Function
Xylose	Fermentable sugar
Lysine	Decarboxylation substrate
Sodium deoxycholate	Selectivity
Phenol red	pH indicator
Sodium thiosulfate + Ferric ammonium citrate	H₂S detection

Preparation

1. Suspend 55 g/L.
2. Heat gently to dissolve.
3. Do NOT autoclave.
4. Cool and pour plates.

Observe Colonies

Organism	Expected
Salmonella Typhimurium ATCC 14028	Red colonies with black centres
Shigella flexneri ATCC 12022	Red colonies
Escherichia coli ATCC 25922	Yellow colonies

• Confirm isolates biochemically.
• Heavy inoculum may alter reactions.
• Some Proteus produce H₂S resembling Salmonella.

1. MacFaddin.
2. Bailey & Scott.
3. CLSI M22.